Melt: 1 1/2 tablespoons butter in a 1 quart heavy saucepan over moderate heat. Whisk in flour and cook, whisking constantly, for 2 minutes to make a roux. Add stock in a slow stream, whisking constantly, and bring to a boil. Reduce heat and simmer, whisking occasionally for 3 minutes. Remove from heat and cover to keep warm.
Pat beef dry and season well with salt and pepper. heat 1 tablespoon oil in a 12 inch heavy skillet over moderately high heat until foam subsides. Add beef in 2 batches and cook, turning once, until browned on both sides but still pink inside, about 1 minute per batch. With a slotted spoon, transfer to a plate.
Add remaining 1 tablespoon each butter and oil to skillet and heat until hot but not smoking. Add shallots and cook, stirring occasionally, until golden brown, about 3 minutes. Add mushrooms and cook, stirring occasionally, until liquid mushrooms give off has evaporated and mushrooms are browned, about 8 to 10 minutes.
Return meat, with its juices, to skillet and stir to combine, then transfer to a platter.
Reheat sauce over low heat, then whisk in sour cream, mustard, dill, 1/2 teaspoon salt and 1/4 teaspoon pepper (do not let boil). Pour sauce over beef and serve with noodles.